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Carbohydrate Polymers 72 (2008) 240–247

Preparation and properties of chitosan-coated NPK compoundfertilizer with controlled-release and water-retention

Lan Wu a,b, Mingzhu Liu a,*

a Department of Chemistry and State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, PR Chinab College of Chemical Engineering, Northwest Minorities University, Lanzhou 730030, PR China

Received 22 April 2007; received in revised form 5 August 2007; accepted 8 August 2007Available online 6 September 2007

Abstract

To improve the utilization of fertilizer and water resource at the same time, a new type chitosan-coated nitrogen, phosphorus andpotassium compound fertilizer with controlled-release and water-retention (CFCW) was prepared, which possessed the three-layer struc-ture. Its core was water-soluble nitrogen, phosphorus and potassium (NPK) fertilizer granular, the inner coating was chitosan (CTS), andthe outer coating was poly(acrylic acid-co-acrylamide) (P(AA-co-AM)) superabsorbent polymer. Atomic absorption spectrophotometerand element analysis results showed that the product contained 7.98% potassium (shown by K2O), 8.14% phosphorus (shown by P2O5),and 8.06% nitrogen. The synthesis conditions of inversion suspension polymerization were studied systematically. The water absorbencyof the product was 70 times its own weight if it was allowed to swell in tap water at room temperature for 90 min. The nutrients slowrelease behaviors of the CFCW in soil and water-retention capacity of the soil with CFCW were investigated. A possible slow releasemechanism was proposed and the release rate factor K and the release exponent n of nutrients in CFCW was calculated. This productwith good controlled-release and water-retention capacity, being degradable in soil and environment-friendly, could be especially usefulin agricultural and horticultural applications.� 2008 Published by Elsevier Ltd.

Keywords: Chitosan; P(AA-co-AM) superabsorbent; Controlled-release; Water-retention; NPK compound fertilizer

1. Introduction

The growth of plants and their quality are mainly afunction of the quantity of fertilizer and water. So it is veryimportant to improve the utilization of water resources andfertilizer nutrients. However, about 40–70% of nitrogen,80–90% of phosphorus, and 50–70% of potassium of theapplied normal fertilizers is lost to the environment andcannot be absorbed by plants, which causes not only largeeconomic and resource losses but also very serious environ-mental pollution. Recently, the use of slow release fertiliz-ers is a new trend to save fertilizer consumption and tominimize environmental pollution (Akelah, 1996; Jar-osiewicz & Tomaszewska, 2003).

0144-8617/$ - see front matter � 2008 Published by Elsevier Ltd.

doi:10.1016/j.carbpol.2007.08.020

* Corresponding author. Tel.: +86 931 8912387; fax: +86 931 8912582.E-mail address: mzliu@lzu.edu.cn (M. Liu).

Slow release fertilizers are made to release their nutrientcontents gradually and to coincide with the nutrientrequirement of a plant. These fertilizers can be physicallyprepared by coating granules of conventional fertilizerswith various materials that reduce their dissolution rate.The release and dissolution rates of water-soluble fertilizersdepend on the coating materials. At present, coating mate-rial’s degradability is an important focus of the research inthis field because of the renewed attention towards environ-mental protection issues (Ge et al., 2002; Shavit, Reiss, &Shaviv, 2002).

Chitosan is a highly deacetylated derivative of chitin,one of the most abundant natural and biodegradable poly-mers. It has been widely applied in the biomedical, pharma-ceutical, and agricultural fields. In many of theseapplications chitosan is extremely attractive due to its bio-degradability, biocompatibility, and nontoxicity (Huacai,Wan, & Dengke, 2006). Therefore, the soluble fertilizer

L. Wu, M. Liu / Carbohydrate Polymers 72 (2008) 240–247 241

coated by carbohydrate polymers, such as chitosan, wouldbe an ideal slow release formulation.

Superabsorbents are loosely crosslinked hydrophilicpolymers that can absorb and retain aqueous fluids up tothousands of times their own weight, and the absorbedwater is hardly removable even under some pressure.Because of their excellent characteristics, superabsorbentpolymers had been widely used in agriculture and horticul-ture (Zhang, Li, & Wang, 2006). Recently, research on theuse of superabsorbent polymers as water managing materi-als for the renewal of arid and desert environment hasattracted great attention, and encouraging results havebeen observed as they can reduce irrigation water con-sumption, and lower the death rate of plants (Bajpai &Giri, 2003; Lin, Wu, Yang, & Pu, 2001). The optimizedcombination of slow release fertilizers and superabsorbentpolymers may improve the nutrition of plants, mitigate theenvironmental impact from water-soluble fertilizers, reducewater evaporation losses, and lower the frequency ofirrigation.

On the basis of the above background and our previousstudies on superabsorbent polymers (Chen, Liu, & Ma,2005; Qi, Liu, Chen, & Liang, 2007) and slow release fertil-izers (Guo, Liu, Zhan, & Wu, 2005; Liang & Liu, 2006), weprepared in this study chitosan-coated NPK compoundfertilizer with controlled-release and water-retention(CFCW), whose inner coating was chitosan (CTS), andthe outer coating was poly(acrylic acid-co-acrylamide)(P(AA-co-AM)) superabsorbent polymer. The product weprepared not only has controlled-release property but alsocould absorb water and preserve the soil moisture at thesame time. These were significant advantages over the nor-mal slow release fertilizers and superabsorbent polymersfor agriculture, which general have only a controlled-release property or water-retention function. In addition,the chitosan in the first layer of the coating material wasa biodegradable material and the copolymer of AA andAM of the outer coating material can also be degraded insoil, so the coating materials were not harmful to the soil.The aim of the present work is to reveal the synthesis con-ditions of inversion suspension polymerization, controlled-release, and water-retention properties of the CFCW.

2. Experimental

2.1. Materials

Chitosan (CTS, degree of deacetylation is 0.9, averagemolecular weight is 6.0 · 105) was kindly provided by Jinx-ing Biochemical Co (Zhejiang, China). Acrylic acid (AA,distilled under reduced pressure before use) was suppliedby Beijing Eastern Chemical Works (Beijing, China).Acrylamide (AM), ammonium persulfate (APS, recrystal-lized from distilled water before use) and N,N 0-methylene-bisacrylamide (NNMBA, used as received) were suppliedby Shanghai Chemical Reagent Factory (Shanghai, China).NPK compound fertilizer was industrial grade.

2.2. Preparation of CFCW

NPK compound fertilizer granule, previously sieved toabout 2 mm in diameter was placed into a rotary drum,and the CTS powder was stuck on the granules by meansof epoxy dissolved in acetone. The adhesive was appliedby spraying at regular time intervals. The process was fin-ished until compact and homogeneous coating formed onfertilizer granule. The coated granules were dried to a con-stant mass at room temperature for 8 h. Then the CTS-coated NPK compound fertilizer granules were obtained.

The CTS-coated NPK compound fertilizer granules(5 g) were added into a flask equipped with a mechanicalstirrer, a condenser, and a drop funnel. A certain amountof carbon tetrachloride, polyethylene glycoloctyl phenylether (OP), and sorbite anhydride monostearic acid ester(Span-80) were added into the flask. The temperature wasraised to 65 �C using a water bath while the contents wereconstantly stirred. After that, a certain amount of mixedsolution of partially neutralized acrylic acid (AA, byammonia), acrylamide, N,N 0-methylenebisacrylamide(NNMBA) solution, and ammonium persulfate (APS) solu-tion was slowly dropped into the flask. The mixture was fil-tered to remove the carbon tetrachloride after being stirredfor 2 h at 65 �C and then dried in a 70 �C oven to obtainthe final product, chitosan-coated NPK compound fertil-izer with controlled-release and water-retention (CFCW).

2.3. Component and structural analysis of CFCW

Contents of nitrogen, phosphorus, and potassium in theCFCW were determined by an element analysis instrument(Germany Elemental Vario EL Crop., model 1106) andatomic absorption spectrophotometer (American TJACrop., model 1100-B). The structure of CFCW was alsosubjected to a scanning electron microscopy (SEM) study.They were split into two halves, and the fractions obtainedwere adhered to sample holders with carbon LIT-C glue.The sample was metal coated with a layer of gold andobserved in a JSM-5600LV SEM manufactured in Japan.

2.4. Characterization of the outer coating material of CFCW

by FTIR

The outer coating material of CFCW, P(AA-co-AM)was characterized by a Fourier Transform Infrared (FTIR)spectrophotometer (American Nicolet crop., model 170-SX).

2.5. Measurement of water absorbency of CFCW

A sample (1 g) of the CFCW was immersed into a cer-tain amount of tap water and allowed to soak at room tem-perature for 90 min. The swollen CFCW was filtratedthrough an 80-mesh sieve to remove non-absorbed waterand weighted. The water absorbency was calculated usingthe following equation:

242 L. Wu, M. Liu / Carbohydrate Polymers 72 (2008) 240–247

WA ¼ M �M0

M0

ð1Þ

Here M and M0 denote the weight of swollen CFCW andthe weight of the dry CFCW, respectively, and WA is thewater absorbency per gram of dried CFCW.

2.6. Slow release behavior of CFCW in soil

To study the slow release behavior of CFCW in soil, thefollowing experiment was carried out: 1 g of CFCW waswell mixed with 200 g of dry soil (below 26-mesh) and keptin a 200 ml beaker properly covered and incubated for dif-ferent periods at room temperature. Throughout the exper-iment, the soil was maintained at 30 wt% water-holdingcapacity by weighing and adding distilled water if neces-sary, periodically. After 1, 3, 5, 10, 15, 20, 25 and 30 daysincubation period, the remaining granular CFCW in thebeakers were picked out and washed with distilled water,and then dried at room temperature overnight to estimatethe contents of N, P and K. For eight measurements, eightbeakers were prepared at the same time. The remainingamount of N, P and K was estimated by the Kjeldahlmethod of distillation (Abraham & Rajasekharan, 1996)and atomic absorption spectrophotometer, respectively.

The release results were analyzed by using an empiricalequation to estimate the value of n and K as follows (Al-Zahrani, 1999; Peng, Zhang, & Kennedy, 2006):

Mt

M¼ Ktn or log

Mt

M

� �¼ logðKÞ þ n logðtÞ ð2Þ

where Mt/M is the released fraction at time t, n is the re-lease exponent, and K is the release factor. From the slopeand intercept of the plot of log(Mt/M) versus log(t), kineticparameters n and K were calculated.

2.7. Largest water-holding ratio of the soil

The sandy soil used in this study was representative ofthe area of Lanzhou, which lies in the northwest of Chinaand is a dry and semi-desert region. A 2 g sample of CFCWwas well mixed with 200 g of dry soil (below 26-mesh) andplaced in a PVC tube of 4.5 cm diameter; the bottom of thetube was sealed by nylon fabric (with an aperture of0.076 mm) and weighed (W1). The soil samples were slowlydrenched by tap water from the top of the tube until thewater seeped out from the bottom. After there was no seep-ing water at the bottom, the tube was weighed again (W2).A control experiment, viz., with no CFCW, was also car-ried out. The largest water-holding ratio (W%) of the soilwas calculated using the following equation:

W % ¼ 100� ðW 2 � W 1ÞW 2 � W 1 þ 200

ð3Þ

2.8. Measurement of the water retention of CFCW in soil

Two grams of CFCW was well mixed with 200 g of drysandy soil (below 2 mm in diameter) and kept in a beakerand then 200 g of tap water was slowly added into the bea-ker and weighed (W1). A controlled experiment, i.e., with-out CFCW, was also carried out. The beakers weremaintained at room temperature and were weighed every4 days (Wi) over a period of 28 days. The water retentionratio (WR%) of soil was calculated using the followingequation:

WR% ¼ W i

W 1

ð4Þ

3. Results and discussion

3.1. The structure and characteristics of CFCW

The structure of CFCW fertilizer granule was thethree-layer, which the core is NPK compound fertilizer(52.8 wt%), the inner coating is CTS (12.3 wt%) and theouter coating is P(AA-co-AM) superabsorbent polymer(34.9 wt%), respectively (see Fig. 1). Element analysisand atomic absorption spectrophotometer results showedthat N, P, and K contents of CFCW were 8.06, 8.14%(shown by P2O5), and 7.98% (shown by K2O) wt%. Theaverage weight of the CFCW fertilizer granules was 0.26g. The average diameters of dry CFCW and swollenCFCW granules in tap water were 2.3 and 12.4 mm,respectively.

3.2. FTIR analysis of P(AA-co-AM) (the outer coating

material)

The infrared spectrum of P(AA-co-AM) shows in Fig. 2.The observed peaks are at 3439 cm�1, corresponding toN–H stretching of acrylamide unit, 2925 cm�1, correspond-ing to the C–H stretching of acrylate unit, 1701 cm�1, cor-responding to the stretching of C@O in acrylate unit,1635 cm�1, corresponding to the carbonyl moiety of theacrylamide unit, 1075 cm�1, corresponding to the –CO–O–stretching of acrylate unit. These results proved thatthe outer coating was P(AA-co-AM) superabsorbentpolymer.

3.3. Influence of the parameters on water absorbency

The key properties of superabsorbent polymers are thewater absorbency and the elastic modulus of the swollencrosslinked gel, both of which are related to the crosslink-ing density of the network. To improve the water absor-bency capacity of the CFCW, various reactionparameters are employed. The complete details of the influ-ence of the reaction parameters on water absorbency in tapwater, such as the content of AM, crosslinking agent, initi-ator, neutralization degree, are given below.

P

N

K

CTS(the inner coating)P(AA-co-AM)(the out coating)

NPK Fertilizer(the core)

Fig. 1. The cross-section schematic view (a) and photograph (b) of CFCW fertilizer granule.

4000 3500 3000 2500 2000 1500 1000 500

1635

1075

1701

2925

3439

%Tr

ansm

ittan

ce

Wavenumbers (cm-1)

Fig. 2. The FTIR spectrum of P(AA-co-AM).

L. Wu, M. Liu / Carbohydrate Polymers 72 (2008) 240–247 243

3.3.1. Effect of the amount of AM on water absorbencyHydrophilic group plays an extremely important part in

water absorbency. In the P(AA-co-AM) polymeric system,the ratio of different hydrophilic groups can be adjusted bychanging the proportion of AA–AM. The effect of theamount of AM on water absorbency of CFCW is shown

5 10 15 20 25 30 35 4030

40

50

60

70

Wat

er a

bsor

benc

y (g

/g)

WAM/WAA(%)45

Fig. 3. The effect of the amount of AM on water absorbency. Neutral-ization degree, 70%; initiator, 0.91 wt%; crosslinking agent, 0.28 wt%;reaction temperature, 65 �C.

in Fig. 3. The water absorbency increases as the amountof AM rise from 10 to 20 wt% and decreases when theamount of AM is greater than 20 wt%. It is known thatAM is a nonionic monomer. It almost does not ionize insolution, so the ions in solution have little influence on it,which would improve the salt-resistance ability of CFCW.Therefore, with the increase of the amount of AM, waterabsorbency would increase with that. On the other hand,–COO� group has better hydrophilic ability than–CONH2, so water absorbency would decrease with thefurther increase of the amount of AM. In other words,the maximum in the dependence of water absorbency onthe amount of AM could be attributed to the cooperationeffect between the salt-resistance property of the –CONH2

group and the high hydrophilic ability of the –COO�

group.

3.3.2. Effect of the amount of initiator on water absorbency

Fig. 4 shows the effect of the amount of initiator onwater absorbency of CFCW. The water absorbencyincreases as the amount of initiator rise from 0.45 to

0.4 0.6 0.8 1.0 1.230

40

50

60

70

80

Wat

er a

bsor

benc

y (g

/g)

Winitiator/WAA (%)1.4

Fig. 4. The effect of the amount of initiator on water absorbency. AM,20 wt%; neutralization degree, 70%; crosslinking agent, 0.28 wt%; reactiontemperature, 65 �C.

244 L. Wu, M. Liu / Carbohydrate Polymers 72 (2008) 240–247

0.91 wt% and decreases when the amount of initiator isgreater than 0.91 wt%. According to the theory of Flory(Flory, 1953), the molecular weight in free radical polymer-ization will decrease with an increase of initiator concentra-tion. With the decrease of the molecular weight, the relativeamount of polymer chain ends increases. As reported in aprevious study (Kiatkamjornwong & Wongwatthaasatien,2004), the polymer chain ends do not contribute to thewater absorbency. Therefore, the increase of the amountof initiator is responsible for the decrease in water absor-bency. However, further decreases in the amount of initia-tor below the optimum values are accompanied by adecrease in water absorbency. This result may be attributedto a decrease in the number of radicals produced as theamount of initiator decreases. The network cannot formefficiently with a small number of radicals in the free radi-cal polymerization that result in the decrease of the waterabsorbency.

3.3.3. Effect of the amount of crosslinking agent on water

absorbency

The crosslinking agent plays an important role in theformation of three-dimensional network structures perma-nently in the polymerization process. This is also a prom-ising factor directly affecting the water absorbency ofCFCW. Fig. 5 shows the water absorbency in tap wateras a function of the amount of crosslinking agent. Itcan be found that there exists a maximum, and the high-est water absorbency occurs at 0.28 wt% crosslinkingagent. When the crosslinking agent is lower than 0.28wt%, the water absorbencies decrease because of theincrease of soluble materials. On the other hand, highercrosslinking agent results in the generation of more cross-link points, which in turn cause the formation of an addi-tional network and decrease the space for holding water.These results are in conformity with the theory of Flory(Flory, 1953).

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.5030

40

50

60

70

Wat

er a

bsor

benc

y (g

/g)

Wcrosslinking agent /W (%)AA

Fig. 5. The effect of the amount of crosslinking agent on waterabsorbency. AM, 20 wt%; neutralization degree, 70%; initiator,0.91 wt%; reaction temperature, 65 �C.

3.3.4. Effect of neutralization degree of AA on water

absorbency

The neutralization degree of AA dependence of waterabsorbency of CFCW is presented in Fig. 6. Neutralizationdegree is defined as the molar percentage of carboxyls inAA neutralized by ammonia. It can be found that thereexists a maximum and the highest water absorbency occursat neutralization degree = 70% . It was expected that theactivity of acrylic acid is higher than acrylate, when theneutralization degree was lower than 70%, the lower neu-tralization degree was faster than the rate of the polymeri-zation, the content of the oligomers would increase, andthe content of acrylate would decrease, so the soluble partincreased and the electrostatic repulsion between theattached carboxylate anions decreased, so that the stretch-ing extend of hydrogel network decreased, and this resultedin water absorbency increasing with the increase of neutral-ization degree. It was expected that the activity of acrylicacid is higher than acrylate so the lower the neutralizationdegree was, the faster the polymerization rate was, and thehigh polymerization rate would result the increase ofthe content of oligomers (the soluble part) in CFCW. Atthe same time, the charge density of the network wouldincrease with the increase of neutralization degree, and thiswould result the increase of the stretching extend of thehydrogel network, so it is easy to understand the increaseof water absorbency as the neutralization degree increasedfrom 0 to 70%.

3.4. Slow release behavior of CFCW

One of the most important characteristics of CFCW weprepared was its controlled-release property. Fig. 7 repre-sents the N, P and K slow release behaviors of CFCW insoil. It could be seen from Fig. 7 that the N, P, and K inCFCW released 14.7, 10.9 and 12.4% on the 3rd days,19.2, 14.3 and 16.7% on the 5th days, and 79, 62, and

40 50 60 70 80 90

40

50

60

70

Wat

er a

bsor

benc

y (g

/g)

Neutralization degree (%)

Fig. 6. The effect of neutralization degree of AA on water absorbency.AM, 20 wt%; initiator, 0.91 wt%; crosslinking agent, 0.28 wt%; reactiontemperature, 65 �C.

0 10 15 20 25 300

20

40

60

80

Rel

ease

d (%

)

Time (day)

abC

5

Fig. 7. Release behaviors of nitrogen (a), potassium (b), and phosphorus(c) in soil.

0.0 0.4 0.8 1.2 1.60.0

0.5

1.0

1.5

2.0

log

(Mt/M

)

log (t)

abC

Fig. 8. Plot of release fractions of nutrient against time: nitrogen (a),potassium (b), and phosphorus (c).

Table 1The release factors (K), release exponents (n), and determination coeffi-cients (r2) following linear regression of release data of nutrients fromCFCW in soil

Nutrients n K r2

N 0.52 7.48 0.9851K 0.54 6.19 0.9905P 0.57 5.20 0.9899

L. Wu, M. Liu / Carbohydrate Polymers 72 (2008) 240–247 245

69% on the 30th days, respectively. With the sum of nutri-ents release lower than 15% on the 3rd day and not above75% on the 30th day, this indicated that the slow releasecharacter of the CFCW we prepared agreed with the stan-dard of slow release fertilizers of the Committee of Euro-pean Normalization (CEN) (Trenkel, 1997), and alsoindicated that the CFCW had an excellent controlled-release property.

The nutrient release mechanism of CFCW in soil can bedescribed by the following steps: (1) The outer P(AA-co-AM) layer is slowly swollen by the water in soil and trans-forms to hydrogel. A dynamic exchange between the freewater in the hydrogel and the water in soil will develop(David & Mark, 1994; Smyth, Francis, & Vincent, 1998).(2) When the free water in the P(AA-co-AM) layermigrates to the middle layer, i.e. CTS coating, the waterwill penetrate the inner coating slowly in the initial stageand the dissolved NPK compound fertilizer in the core.In this stage, diffusion would be the release rate-limitingstep. (3) Under the effect of water, ions and microorgan-isms existing in the soil, the middle CTS layer will slowlydegrade in the last stage and continue to dissolve NPKcompound fertilizer. In this stage, degradation rate deter-mines the nutrients release rate. (4) The dissolved NPKcompound fertilizer diffuses out the CTS layer and entersinto the P(AA-co-AM) layer, and then releases into the soilthrough the dynamic exchange of free water.

The n value is an empirical parameter characterizing therelease mechanism (Shaviv, 2000). On the basis of the dif-fusion exponent, an n value of 0.5 indicates the nutrientrelease mechanism approaches to a Fickian diffusion con-trolled release, whereas n equal to 1.0 indicates the nutrientrelease mechanism approaches to zero-order release. The n

value from 0.5 to 1.0 is a nutrient release mechanism fornon-Fickian diffusion or chain relaxation control release.From the plot of log(Mt/M) versus log(t) (Fig. 8), therelease exponent (n) and release factor (K) have been calcu-lated. Table 1 summarizes the values for N, P, and K

release from CFCW in soil. The n value is in the range from0.52 to 0.57. Therefore, the nutrients release mechanism isnon- Fickian diffusion, and is controlled by a combinationof diffusion of fertilizer from coating material and degrada-tion rate of CTS layer. Additionally, due to the existing ofmany kinds of ions in soil solution, the swelling degree ofP(AA-co-AM) is less in soil than that in distilled water,then the diffusion of soluble fertilizer in it would be diffi-cult, which also contribute to slow release of nutrients fromCFCW (Li & Wang, 2005).

3.5. The largest water-holding ratio of the soil

Besides its controlled-release property, another one ofthe most important characters of the CFCW we preparedis its water-retention capacity or, in other words, its effec-tive utilization of water in arid and desert regions to trans-form them into ‘‘green and fertile lands’’. It was reported(Bakass, Mokhlisse, & Lallemant, 2002) that the use ofsuperabsorbent polymer in the agricultural field couldincrease the largest water-holding capacity and water reten-tion capacity of soil. Therefore, the experiments to test thewater-holding capacity and water retention behaviors ofsoil with CFCW were performed. The experiment resultsof the largest water-holding ratio of the soil indicated thatthe largest water-holding ratio of the soil without CFCWwas 30.17%, and that of the soil with CFCW was 40.35%(the mass ratio of CFCW to soil was 1:100), 10.18% higher

246 L. Wu, M. Liu / Carbohydrate Polymers 72 (2008) 240–247

than the former. This showed that the CFCW we preparedstill had excellent water absorbency in soil, could obviouslyimprove the water-holding capacity of the soil, and couldefficiently store rainwater or irrigation water resources.This was one of the significant advantages over the normalslow release fertilizers.

3.6. Water retention behavior of CFCW in soil

The most important application of CFCW is for agricul-ture and horticulture, especially for saving water in dry anddesert regions and for accelerating plant growth. So, it isnecessary to investigate the water-retention ability ofCFCW in soil. Fig. 9 shows the water-retention behaviorsof the soil with (a) and without (b) CFCW. From Fig. 9we find that the addition of CFCW to soil could obviouslyincrease the water-retention and decrease the water evapo-ration. The water retention ratio of soil without CFCWhad only remained 12.4 and 2.6 wt% on the 10th and20th days, respectively, while that of the soil with CFCWwas 24.7 and 15.5 wt%, respectively. After 30 days, the soilwithout CFCW had already given off all the water, whilethe soil with CFCW still had 7.8 wt% water-retentionratios.

From this study, it could be inferred that CFCW hadgood water-retention capacity in soil, and that withCFCW use water can be saved and managed so that theycan be effectively used for the growth of plant. These werethe significant advantages over the normal slow releasefertilizers, which always only had a controlled-releaseproperty. The reason was that the outer coating ofCFCW could absorb and store a large quantity of thewater in soil, and allow the water absorbed in it to beslowly released with the decrease of the soil moisture.Simultaneously, nutrition could also be released slowlywith the water. Therefore, the swollen CFCW was justlike an additional nutrient reservoir for the plant–soil sys-

0 10 15 20 25 300

10

20

30

40

Wat

er re

tent

ion

(%)

Time (day)

a

b5

Fig. 9. Water retention behaviors of soil with CFCW (a) and soil withoutCFCW (b).

tem, and thus could increase the utilization efficiency ofwater and fertilizer at the same time. Furthermore, aswe known, the chitosan in the first layer of the coatingmaterial was a biodegradable material (Borzacchielloet al., 2001) and the copolymer of AA and AM of theouter coating material can also be degradable in soil(Ye, Zhao, & Zhang, 2004), so the coating materials werenot harmful to the soil. Thus there would be a good usepotentiality in dry-prone regions.

3.7. Morphological analysis

From the SEM of the surface of the CFCW, it can beseen that the surface of CFCW is rugged, which structur-ally increased the surface area of the CFCW. Therefore,when CFCW is dipped in water, it can absorb waterquickly to form a swollen hydrogel, which is responsiblefor the water-retention property of CFCW. The SEM ofthe cross-section of the CFCW shows the three-layersstructure of CFCW. The outer layer is P(AA-co-AM)superabsorbent polymer, which could absorb a largeamount of water. The middle layer is CTS, which servesas a physical barrier for mass transfer, and reduces the rateof water diffusion into the core and the nutrient diffusionoutside the core, this provided the CFCW with a good con-trolled-release property. The inner core is a water-solubleNPK compound fertilizer granule. In summary, the outerP(AA-co-AM) layer enables the CFCW water-retentionproperty, and the middle CTS layer enables the CFCWcontrolled-release property.

4. Conclusions

A chitosan-coated NPK compound fertilizer with con-trolled-release and water- retention (CFCW) was prepared,which possessed the three-layer structure. Its core waswater-soluble NPK fertilizer granular, the inner coatingwas chitosan (CTS), and the outer coating was P(AA-co-AM) superabsorbent polymer. Element analysis andatomic absorption spectrophotometer results showed thatthe N, P and K contents were 8.06, 8.14 (shown by P2O5)and 7.98 (shown by K2O) wt%, respectively. The producthad good slow release property, the nutrients released didnot exceed 75% on the 30th day. The analysis of releaseshowed that nutrient might be released from CFCW in soilby non-Fickian diffusion mechanism. The addition ofCFCW into soil could greatly improve the water holdingability and water retention property of the soil. Moreover,this new approach showed promising in utilizing naturalresource such as chitosan in the production of coatingmaterial, which could significantly reduce the productioncost and make the technique quite environmental friendly.The results of the present work indicate that the CFCWwas a good slow release fertilizer with excellent water-retention capability. Therefore, CFCW would find goodapplication in agriculture and in the renewal of arid anddesert environments.

L. Wu, M. Liu / Carbohydrate Polymers 72 (2008) 240–247 247

Acknowledgement

This work was supported by Special Doctorial ProgramFunds of the Ministry of Education of China (Grant No.20030730013).

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